Split-Brain Research into Hemispheric Lateralisation

• Hemispheric lateralisation = The idea that each hemisphere (half) of the brain is mainly responsible for certain behaviours, processes and activities. This is in contrast with the holistic theory of brain function, which suggests that function is distributed across the whole brain (i.e. is global).
• The right hemisphere controls the left side of the body, and vice versa. Therefore, information which we receive from the left visual field is processed by the right hemisphere, which then coordinates a response to affect the left side of the body.
• Sperry and Gazzaniga (1968) conducted split-brain research on 11 epileptic patients. In order to control their seizures, these patients underwent surgical lesioning of the corpus callosum where information processed by one hemisphere cannot be relayed to the other hemisphere. Therefore, it is possible to expose a single hemisphere to certain stimuli in split-brain patients, and thus infer the functions of each hemisphere.
• The patients had one of their eyes covered, so that information would not be received by both eyes. The stimuli was flashed onto a screen for a second. This prevented both visual fields of the eye being exposed to the information, so only one hemisphere would process it. Therefore, their research was conducted under strictly controlled conditions, through the use of a laboratory experiment.
• first describe what you see = If the stimulus word was exposed to the right visual field, then it would be processed by the left hemisphere and the patient would say the word. This is because the left hemisphere contains the ‘language centres’ of the brain and so allows for speech.
• However, if the same stimulus word was exposed to the left visual field, then it would be processed by the right hemisphere and the patient would write the word using their left hand. This is because the right hemisphere contains the visuo-spatial centres of the brain, allowing for the physical act of writing. The patient would not be able to give a verbal description of the word, because the right hemisphere contains no language centres.
• then matching words or faces = The right hemisphere appeared to dominate the ability to match a list of faces to a given stimulus. This is due to the right hemisphere containing the brain’s visuo-spatial centres, thus allowing for the visual identification and processing of the faces.
• Words presented simultaneously = If two words were presented at the same time, each to one of the visual fields, the patient would say the word presented to the right visual field (processed by the left hemisphere with language centres) and write down the word presented to the left visual field (processed by the right hemisphere and containing visuo-spatial centres).
• Recognizing objects placed into the hands = If an object was placed into the patient’s right hand, they would be unable to identify that it is there, because the information would be processed by the left hemisphere which only has language centres, and no visuo-spatial centres. Therefore, if an object was placed into the patient’s left hand, they would be able to identify the object and choose a similar one from a hidden bag, due to the action of the visuo-spatial centres
• weakness is the lack of control with the sample selection. The epileptic patients had been taking anti-epilepsy medications for extended and different periods of time, which may have affected their ability to recognise objects and match words, due to causing cerebral neuronal changes.
• Secondly, although all patients had undergone a surgery, there may have been differences in the exact procedures e.g. differing extent of the change of the corpus callosum. This would have affected the degree to which the two hemispheres could relay information between themselves. Therefore, these two confounding variables had not been controlled, meaning that the lateralised functions may be examples of unreliable causal conclusions.
• Clearly demonstrated lateralisation of function = Split-brain research was pivotal in establishing the differences in functions between the two hemispheres, and so opposing the holistic theory of brain function. The left hemisphere was demonstrated as being dominant for language tasks, due to containing language centres, whereas the right hemisphere was demonstrated as being dominant for visuo-spatial tasks. Therefore, this suggests that the left hemisphere is the analyser, whereas the right hemisphere is the synthesiser, and so there are marked differences between the two.